Scandinavians have been keeping careful records on themselves for many generations, which has been a boon to social scientists. Here's a new paper on IQ and Family Background that uses IQ data from the IQ test given to Swedish conscripts. Sample sizes are ample (over 20,000 father-son pairs, and hundreds of thousands of brother pairs):

We use a large representative sample of Swedish men to examine both intergenerational [father-son] and sibling [brother] correlations in IQ. Since siblings share both parental factors and neighbourhood influences, the sibling correlation is a broader measure of the importance of family background than the intergenerational correlation. We use IQ data from the Swedish military enlistment tests. The correlation in IQ between fathers (born 1951-1956) and sons (born 1966-1980) is estimated to 0.347. The corresponding estimate for brothers (born 1951-1968) is 0.473, suggesting that family background explains approximately 50% of a person’s IQ. Estimating sibling correlations in IQ we thus find that family background has a substantially larger impact on IQ than has been indicated by previous studies examining only intergenerational correlations in IQ. ...

What is it then that brothers share and is important for their IQ but is uncorrelated with their father’s IQ? An obvious candidate is the mother’s IQ.

Although spouses’ IQ are most likely positively correlated and thus partly capture the same background factors, the combination of father’s and mother’s IQ is likely to raise the explanatory power in an intergenerational equation. Indeed, in a summary of previous estimates based on small and non-representative samples, Bowles & Gintis (2002) report the highest correlation from a study that applies the average of the two parents’ IQ. We doubt though that simply adding mother’s IQ would bring the explanatory power close to what the sibling similarity suggests. For example, attempts to account for the sibling similarity in long-run earnings by means of the education of both parents do not appear to capture much of the sibling similarity (Bj?¶rklund et al. 2008). We hypothesize that very detailed information about parental aspirations, attitudes and parenting practices is needed to account for the large gap between what sibling studies and intergenerational studies suggest about the role of family background factors.

Of course, peer groups are also likely to play an environmental role as well as parents. Brothers are part of the peer group because they tend to interact a lot with each other, especially if they are close in age. Brothers born within 5 years of each other have IQs that correlate at about the 0.50 level, versus about 0.44 for brothers born more than five years apart. The researchers call that difference "marginal," and assert "permanent family and community characteristics are more likely determinants," but they seem pretty interesting to me since they are one factor we can tell (lacking maternal IQ scores) that isn't genetic.

Keep in mind that the higher correlation among brothers closer in age is not just measuring the influence of brothers on each other but of broader environments. Two brothers who are only two years apart are more likely to have, say, both been taken care of by the same grandmother, played with the same cousin, gone to the same school, watched the same TV shows, were supported by similar levels of parental income, and so forth than two brothers who are eight years apart.

Brothers born on the same day (i.e., twins) correlate at 0.65, with that made up of a mixture of identical and fraternal twins.